Stripe rust (yellow rust), caused by Puccinia striiformis f. sp. tritici is one of the most damaging diseases of wheat in Pakistan. Lack of durable resistance in local wheat varieties is the main reason for stripe rust epidemic which could limit yields. The use of genetically resistant wheat varieties is the most economic way of controlling the disease. Evaluation of 135 advance wheat lines for slow yellow rusting was conducted during cropping season [2008][2009] under natural epidemics at field locations of Ayub Agriculture Research Institute, Faisalabad and Cereal Crops Research Institute, Pirsabak (Northwest), Pakistan. Resistance level based on final disease severity (FDS) along with other slow rusting parameters relative area under disease progress curve (rAUDPC) and relative infection rate (RIR) was assessed. A total of 25 wheat lines were identified to potentially have durable resistance with low frequency of disease severity (10-30%) and lower relative AUDPC values (2-66%). Based on the slowing rusting data, RAPD (random amplified polymorphic DNA) primers were used in order to evaluate genetic diversity among 25 lines. Of 20 OPA (Operon series) primers tested, 4 (20%) primers were polymorphic that showed amplification differences among 25 genotypes. OPA-06 and OPA-04 revealed the highest polymorphism (67% and 50%) while OPA-02 and OPA-17 exhibited the lowest polymorphism (33% and 25%) respectively. From the amplification profile, a total of five RAPD markers were obtained in this study. A similarity matrix data depicted that most of these genotypes are genetically very close (60-100%). The 25 advance lines identified from slow rusting evaluation with five RAPD markers may have partial resistance genes and can be used as slow yellow rusting lines with longer field life in Pakistan breeding program.
Wheat production in Pakistan is seriously constrained due to rust diseases and stripe rust (yellow) caused by Puccinia striiformis f. sp. tritici, which could limit yields. Thus development and cultivation of genetically diverse and resistant varieties is the most sustainable solution to overcome these diseases. The first objective of the present study was to evaluate 100 Pakistan wheat cultivars that have been grown over the past 60 years. These cultivars were inoculated at the seedling stage with two virulent stripe rust isolates from the United States and two from Pakistan. None of the wheat cultivars were resistant to all tested stripe rust isolates, and 16% of cultivars were susceptible to the four isolates at the seedling stage. The data indicated that none of the Pakistan wheat cultivars contained either Yr5 or Yr15 genes that were considered to be effective against most P. striiformis f. sp. tritici isolates from around the world. Several Pakistan wheat cultivars may have gene Yr10, which is effective against isolate PST-127 but ineffective against PST-116. It is also possible that these cultivars may have other previously unidentified genes or gene combinations. The second objective was to evaluate the 100 Pakistan wheat cultivars for stripe rust resistance during natural epidemics in Pakistan and Washington State, USA. It was found that a higher frequency of resistance was present under field conditions compared with greenhouse conditions. Thirty genotypes (30% of germplasms) were found to have a potentially high temperature adult plant (HTAP) resistance. The third objective was to determine the genetic diversity in Pakistan wheat germplasms using molecular markers. This study was based on DNA fingerprinting using resistance gene analog polymorphism (RGAP) marker analysis. The highest polymorphism detected with RGAP primer pairs was 40%, 50% and 57% with a mean polymorphism of 36%. A total of 22 RGAP markers were obtained in this study. RGAP, simple sequence repeat (SSR) and sequence tagged site (STS) markers were used to determine the presence and absence of some important stripe rust resistance genes, such as Yr5, Yr8, Yr9, Yr15 and Yr18. Of the 60 cultivars analyzed, 17% of cultivars showed a RGAP marker band for Yr9 and 12% of cultivars exhibited the Yr18 marker band. No marker band was detected for Yr5, Yr8 and Yr15, indicating a likely absence of these genes in the tested Pakistan wheat cultivars. Cluster analysis based on molecular and stripe rust reaction data is useful in identifying considerable genetic diversity among Pakistan wheat cultivars. The resistant germplasms identified with 22 RGAP markers and from the resistance evaluations should be useful in developing new wheat cultivars with stripe rust resistance.
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